Pricing: replace basic pricing with enterprise form

[agjohnson]

As a test, we'll replace the basic plan information on our pricing page with enterprise plan information. Use a email link for now, and later use a mail form through Front.

[ericholscher]

Refs https://github.com/readthedocs/meta/discussions/117#discussioncomment-5534909

Plan is to push forward on the Enterprise inbound as a first step, and then Basic plan changes in another PR.